What Prop do I use

[bkatz]

Hey everyone I have a 1968 johnson 20 hp outboard and I have no idea what kind of prop I should use. The motor is on a 13ft inflatable that will be used for diving so the load could be somewhat heavy when going for a dive (4 guys plus gear). Thanks in advance

 

[bowman316]

Re: What Prop do I use

What pitch prop is on there now?

You need to take the boat out for a test drive with the current prop to know where you need to go.

You will prob want to be somewhere around 10 - 12 inches in pitch, but that is a total guess without a test run.

You really need to know your RPM's at wot.

You go diving up in new england, I would think the water is too cloudy.

 

[Silvertip]

Re: What Prop do I use

At the very top of this forum is a sticky titled "READ THIS FIRST"

 

[bkatz]

Re: What Prop do I use

I wish I could give you all the info I SHOULD know as suggested in the ?sticky? thread but I don?t have any clue. And I don?t even know where to start I am completely new to this. I apologize for my lack of knowledge.

Bowman- Diving in New England is great!! Visibility can be bad at times, but for the most part it isn?t that bad. Invertebrate life is very colorful and there is tons to see Marine life, shipwrecks all kinds of good stuff.

 

[steelespike]

Re: What Prop do I use

No need to apologize.Here's the deal.Look on the prop for a number probably
something like 9 X 12. Diameter X pitch.Pitch is the determining number.
Diameter follows the design.You'll need to get a tach.A tinytach is easy to install and reasonable. Something to tell speed is a big help.A GPS is excellent.
With a few test runs and the information from you we can tell you if you need any prop change.There are some adjustments you can do to improve things as well.Let us know how you make out with your test run.

 

[bkatz]

Re: What Prop do I use

Thanks for the pointers. The boat will not be in the water until mid April so I will test the speed then. For now I will look for the Numbers and let you know. Again thanks for being so helpful.

 

[bkatz]

Re: What Prop do I use

The prop is worn and the number is hard to read but I believe it says 9X10, but I am not 100% sure. ANy thoughts on what prop that is?

 

[steelespike]

Re: What Prop do I use

9 X 10 So we are on the same track.That's 9" X 10", diameter X pitch.Pitch is the critical number. That would be called the "standard" prop.
Most likely you shouild drop to about a 9 X 9 but don't do anything until you test it out. More info later.Just wanted to add my limited experience the
small inflatables are more difficult to get on plane and tend to struggle more with a load.Seems I read somewhere they had special trim tabs for inflatables.